Never have the letters "AMG" meant so much to a Mercedes. Designating the German manufacturer's in-house tuning arm, they have come to mean cars with equal parts performance and pedigree. The CL65 AMG coupe is the most pointed example of that reputation yet, as it is not only one of the most expensive and exclusive Mercedes ever built, it's also one of the most powerful.

Based on the standard CL600 (itself a 12-cylinder version of the V8-powered CL500), the CL65 delivers a level of performance matched only by the world's most exotic sports cars. Unlike its peers, however, this coupe doesn't wear its performance credentials on its sleeve, as the AMG stylists retained the elegant and understated design of the standard CL. Whether you see it as the ultimate sleeper or the proverbial iron fist in a velvet glove, make no mistake about it, this is one of the most ridiculously fast and luxurious coupes ever offered to the American public.

Although AMG's engineers are quick to tout the car's overall performance package, there's no getting around the fact that the car's primary source of intrigue lies under its hood. With 6.0 liters of displacement and twin turbochargers, the CL65's V12 engine is the most powerful production engine ever offered by the German automaker. Each engine is hand-built by a single specialist who then signs his name to the carbon-fiber engine cover. Producing a maximum of 612 horsepower and 723 pound-feet of torque, the aluminum V12 generates nearly twice the power of a standard Porsche 911 and slots the CL65 in the very exclusive club of 500-plus-hp production cars.

The heart of any AMG-modified car, the CL65's twin-turbo V12 generates 612 hp and 723 lb-ft of torque. Look closely and you can see the plate signed by the specialist who assembled the engine himself.

Turn the key and the starter winds for a good second or two before the engine swells to life with a smooth cascade of bass that only a V12 can muster. Thanks to an AMG-specific sports exhaust system, the CL65 adds a subtle snarl to the typical 12-cylinder soundtrack that gives you a hint of its potential. Pull away gently and you would never know that you're behind the wheel of a machine capable of hurling itself to 60 mph in just 4.5 seconds. Under part throttle, the nearly limitless power remains masked behind the refined workings of the AMG-tuned five-speed automatic transmission. Unlike so many exotics, the CL feels perfectly content to dawdle around town, delivering smooth, imperceptible shifts and a calm demeanor that belies its extraordinary capabilities.

Dip into the throttle any deeper, however, and you're greeted by an instantaneous surge of thrust that is smooth in its application yet scary in its results. It's not abrupt or violent, just a perfectly orchestrated sequence of spooling turbos, shifting gears and chirping tires. As the battle between the massive rear tires and the electronic traction control rages on beneath you, the car builds speed at a ferocious pace, pushing into triple-digit territory with an ease that makes you wonder how — and why — it's even legal. Get off the throttle and the big coupe settles back into its former life, whisking along in silence while Porsches play catch-up.

Maintaining control of a coupe capable of such velocities falls to a suspension reworked by AMG to provide increased capabilities without compromising ride quality. Revised struts, a reprogrammed Active Body Control system and 19-inch wheels and tires are tasked with keeping the weighty (4,750 pounds) coupe in control. Ample stopping power is provided by an upgraded brake system that features newly developed composite front discs that are not only lighter, but can withstand heat better than conventional materials.

A dashboard button can summon a "sport" mode from the Active Body Control system, but on fast sweeping turns and open stretches of highway we rarely felt the need. The ride quality is about what you would expect — firm but forgiving when it needs to be. There's grip to spare and almost no body roll with the ABC cranked up, but a tight, narrow section of our drive route exposed the CL's undeniable size and heft. It may have sports car speed, but when it comes to attacking your local canyons this may not be the ultimate weapon with which to do so.

Exclusive napa leather covers not only the seats but the dash, console and door panels as well. Nearly all features are standard including the newly upgraded DVD navigation system.

Back in less confining surroundings, we were reminded of the CL65's true mission as a luxury grand tourer. Like all CLs, the interior is lavishly appointed and spacious, with nearly every conceivable comfort feature as standard. AMG-specific additions include sport seats, a roof lined in Alcantara suede and napa leather upholstery that covers the seats, dashboard and door panels. It's an environment that encourages the kind of extended trips that the engine is just begging you to take.

Like all cars of this genre, there's always the question of how much is enough? With the standard CL600 making nearly 500 hp, upgrading to the CL65 seems wholly unnecessary. But as we contemplated this issue on our test-drive, one thing kept reverberating in our minds. It was the 65's engine of course, more specifically, the sound it makes when you abruptly come off the throttle. It's a quick but deep hiccup of sorts that most passengers won't even notice. But to a driver who knows, this momentary cough from the big V12 is a subtle but satisfying reminder that they're driving more than just any Mercedes coupe — this is the Mercedes coupe.